Katsumi Tanimura from the Institute of Scientific and Industrial Research, Osaka University, Japan, is invited by Jelena Sjakste and Nathalie Vast from the LSI research group Théorie de la Science des Matériaux. He will speack about time- and angle-resolved photoemission spectroscopy on ultrafast hot-carrier relaxation in semiconductors: State-resolved dynamics of impact ionization, intervalley scattering, and energy relaxation.

Ultrafast scattering of highly energetic carriers in semiconductors constitutes the key process that determines functional limits and properties of nano-, micro- and opto-electronics, thus being a strategic field in both basic and applied researches in past two decades. However, the dynamics of these processes is yet to be understood on a fundamental level because of a serious methodological limitation of previous studies; to date, hot carrier distributions and their relaxation in energy and momentum spaces have only been inferred indirectly.

More particularly, Katsumi Tanimura will report, using time and angle-resolved photoemission spectroscopy, the direct determination of hot electron distributions and their temporal evolution in the conduction band of GaAs and InSb, prototypical direct-gap semiconductors. With his collaborators, he captures the nascent (as photoexcited) electron distributions unambiguously in energy and momentum spaces, and he elucidates directly, by revealing femtosecond dynamics, ultrafast hot electron processes of quasi-thermalization, intervalley scattering, energy relaxation and impact ionization. The present results not only provide definitive answers to important open questions, but also reveal new features of ultrafast relaxation of hot electrons in direct-gap semiconductors. Thus Katsumi Tanimura and his collaborators describe breakthrough measurements that provide much greater understanding of carrier dynamics in semiconductors.